1. Field of the invention
The invention relates to a plasma processing method and plasma processing apparatus for a sample, the method and apparatus being suitable to suppressing damage, due to increase of plasma potential, to the inner wall of a vacuum process container of a semiconductor surface processing apparatus utilizing plasma such as a plasma etching apparatus.
2. Description of the related art
A method of holding an object with electrostatic action is used, in particular, to transfer a wafer in a semiconductor manufacturing apparatus, and to fix a wafer during various processes. As compared to mechanical holding methods using clamps or the like, the method using electrostatic force, known as electrostatic chuck, has more advantages such as being free from wafer contamination due to contact and easy to control wafer temperature because the wafer is attached with the entire surface of its rear side. In this way, electrostatic chuck has many advantages as a method of holding a wafer. For this reason, it is widely used for wafer processing electrodes in dry etcher, CVD or other apparatus. In general, electrostatic chuck systems include the monopole configuration that typically provides a single electrode subjected to a chuck voltage, and the dipole configuration that provides two or more electrodes subjected to chuck voltages with generally different polarities.
In the conventional art described above, a processing apparatus for plasma etching a semiconductor wafer, for example, has the following problems. As processed devices continue to shrink in recent years, there arises the need to eliminate as much of trace heavy metals contaminated in the processing plasma as possible. In this respect, surface insulating material such as high-purity quartz, alumina ceramics, or surface anodized aluminum is widely used instead of conventional conductive material such as SUS. More specifically, the inner wall surface of the process container exposed to plasma is typically covered with insulating material having high plasma resistance, which allows plasma in the process container to be nearly floating with respect to direct current. In this environment, for the purpose of electrostatic chuck, a positive chuck voltage, for example, is applied to the electrode for mounting a semiconductor wafer, and plasma is generated for processing the semiconductor wafer. Minute DC leak current then flows into the plasma (electrons flow out of the plasma) via a dielectric coating on the electrode surface and the semiconductor wafer, and consequently charges up the plasma to a higher positive potential.
Various factors are relevant to the value of the leak current. It is known that the leak current significantly depends on the potential difference between the potential of the wafer surface (negatively charged by injection of electrons from the plasma) and the potential on the output side of the electrostatic chuck power supply. When the positive charge up of the plasma is advanced by the leak current and the plasma potential exceeds a certain level, dielectric breakdown of the insulator layer on the inner wall surface of the process container may cause spark-like abnormal discharge, for example, on a portion of the inner wall surface of the process container, particularly on the surface of anodized aluminum material. Such abnormal discharge may affect the discharge stability of plasma. In addition, metal particles scattered by the abnormal discharge may lead to the increase of device defects and metal contamination of the semiconductor wafer.
The foregoing phenomenon is an example where a positive chuck voltage is applied in the monopole configuration. On the other hand, in the dipole configuration, two electrically isolated electrodes, for example, are subjected to voltages of different polarities (e.g., +500 V and −500 V), respectively, for electrostatic chuck action. As with the monopole configuration, leak current occurs from the positive voltage electrode to the plasma. In the same manner as in the monopole configuration, the wafer surface is negatively charged by electrons supplied from the plasma. The value of leak current flowing from the plasma to the negative electrode via the dielectric layer is smaller than that flowing from the positive electrode to the plasma. This advances charge up of the plasma to a higher positive potential and increases the plasma potential, which eventually leads to dielectric breakdown of the insulator layer on the inner wall surface of the chamber.